The recent growth in size semiconductor wafers in integrated circuit fabrication is making it more difficult to obtain the needed degree of uniformity of plasma process rate across the treated surface of the wafer. The process rate may be an etch rate or a deposition rate, for example. Plasma processing tools for processing 300 mm diameter wafers employ a pair of concentric coils (coil antennas), namely an inner coil and an outer coil, to control uniformity of radial distribution of process rate. The RF power levels applied to the different coils are selected to counteract non-uniformities. For 450 mm diameter wafers, we feel that three separately powered RF coils are needed. Separate adjustment of the different RF power levels delivered to the three different coils would provide finer control of uniformity. Impedance matching is required for ail three coils. The problem is that operation of the impedance match and control of the different power levels delivered to the different coils is unpredictable and complicated by internal resonances and mutual inductances among the three coils and between the coils and the plasma. A related problem is that varying the three different power levels in an effort to improve uniformity may cause the system to enter resonant modes, in which plasma instabilities can arise. Resonances also cause very high RF voltages to appear on the coils and their circuit elements, which can cause arcing.